The invention relates generally to the field of marine geophysical survey systems. More particularly, the invention relates to “stretch sections” used in sensor streamers in marine geophysical survey systems to reduce towing noise.
Marine geophysical survey apparatus known in the art include arrays of sensors, such as electromagnetic and/or seismic sensors disposed in structures adapted to be towed by a survey vessel through a body of water, such as a lake or the ocean. Such structures are known as “streamers” or “sensor streamers.”
Streamers are essentially long cables, typically made up of a plurality of segments of about 75 to 150 meters length each. A streamer may include 100 or more such segments coupled end to end to form the complete streamer. Seismic and/or electromagnetic sensors may be disposed along the length of the streamer at spaced apart locations, typically within a receptacle formed in a buoyancy device. Electrical and/or optical cables may extend along the length of the streamer and may be coupled to the sensors so as to transmit signals generated by the sensors. The signals are communicated over the cable(s) to a recording device, which may be on the survey vessel or at another location such as onboard another vessel. Each streamer segment typically includes a combination mechanical and electrical/optical coupling at each of its axial ends so that the streamer segment can be coupled to another such streamer segment or to a “lead in” cable coupled to the survey vessel. The coupling transfers axial force from segment to segment and ultimately to the survey vessel through the lead in cable. Each coupling may be direct, or may be made through a “stretch section.”
In a typical marine geophysical survey system, one or more streamers made as described above may be towed behind the survey vessel in the water. In survey systems having more than one streamer, the streamers are typically laterally separated from each other by coupling their forward ends at spaced apart positions to a “spreader cable” that extends transversely to the direction of motion of the survey vessel.
A particular issue that concerns marine geophysical survey systems known in the art is noise created by movement of the water past the lead in cable and the spreader cable, and due to acceleration imparted to the streamers as a result of currents in the water and other factors related to friction between the streamer(s) and the water. One device known in the art for reducing transmission of such noise between the lead in cable and the streamer is known as a “stretch section.” For example, stretch sections are described in U.S. Pat. No. 7,184,366 issued to Harrick et al., which is herein incorporated by reference. Stretch sections known in the art are effective in reducing vibratory noise in a range of frequencies, e.g., 20 Hz and above, that are detected by seismic sensors when such are the type of sensors used in the streamers. Such stretch sections have devices, e.g., shock cords, to enable a degree of isolation of acceleration between the components connected by the stretch section. Shock cords may be self damping, however separate damping devices may also be used in stretch sections known in the art.
Marine electromagnetic sensor streamers typically include electromagnetic sensors, such as spaced apart electrode pairs, wire loops or coils, and/or magnetometers to detect naturally occurring electromagnetic fields (magnetotelluric fields) or electromagnetic field components resulting from imparting electromagnetic fields in the body of water and in the formations below the water bottom. Motion induced vibratory noise in such cases may occur in frequency ranges below 10 Hz (e.g., on the order of a vessel swell frequency of around 0.1 Hz). Such noise may impact the electromagnetic field measurement channels and/or measurements of in-streamer motion (often determined with accelerometers).
It is desirable to have a stretch section which can be used in connection with marine electromagnetic sensor streamers as well as with seismic sensor streamers. It is also desirable to have stretch sections with adjustable spring constant and/or damping coefficient to optimize the noise reduction provided by the stretch section, depending on the type of streamer used and on the various conditions in the body of water at the time a geophysical survey is conducted.